Evaluation of Three Models that Describe Soil Water Retention Curves from Saturation to Oven Dryness

Measuring a soil water retention curve (SWRC) with the pressure plate device is generally limited to matric suctions <= 1500 kPa. A few models have been proposed to describe the SWRC from saturation to oven dryness using measurements in the pressure plate matric suction range. The development and validation of these models were mostly based on a limited set of published measurements, and in general, the models have not been validated by independent data from additional soils. In this study, we tested the hypothesis that these models were able to predict the complete SWRC from a limited range of water retention data or by using water retention parameters available from the literature. Soil water retention measurements from saturation to oven dryness were conducted on disturbed, repacked soil samples of various textures using the pressure plate method and the dew point potential technique. When the model parameters were obtained from water retention data in the 0 to 1500 kPa range, the RMSE of water content was approximately 0.01. The RMSE of the Fayer-Simmons model was slightly larger than that of the Webb model and Khlosi model. In addition, the Fayer-Simmons model was sensitive to the data point near the matric suction of 1500 kPa. The Khlosi model produced acceptable results if data sets from 0 to 500 kPa were used for model establishment; larger errors were observed on some soils if the measured data were limited to the 0- to 100-kPa range. All three models could provide reliable results across the entire range of soil water content if measurements in the 0- to 1500-kPa range were available.